Planck reveals Star Formation processes

Image credit: ESA / HFI and LFI Consortia

New images from Planck reveal the gas and dust between the stars and isolate the physical processes at work in our Galaxy. The new images are an eye-catching by-product of a spacecraft designed to look back at the earliest light in the Universe.

Our Galaxy, the Milky Way, is home to billions of stars, laced through with clouds of gas and dust known as the interstellar medium. In visible light most of the newly born stars are hidden by clouds of tiny dust particles dispersed between the stars. When observed at much longer wavelengths, where the Cosmic Microwave Background can be seen, the picture is very different. The dust is no longer a dark shroud, but shines out in its own right, and new aspects of our Galaxy are revealed.

Probing the processes at play

At wavelengths where Planck’s sensitive instruments observe, the Milky Way emits strongly over large areas of the sky. This emission arises primarily from three processes, each of which can be isolated using Planck. At the longest wavelengths, of about a centimetre, the Low Frequency Instrument (LFI) maps the distribution of synchrotron emission due to high-speed electrons interacting with the magnetic fields of our Galaxy. At intermediate wavelengths of a few millimetres, observed by LFI and the High Frequency Instrument (HFI), the emission is dominated by ionized gas being heated by newly formed stars. At the shortest wavelengths of HFI, of around a millimetre and below, Planck maps the distribution of interstellar dust, including the coldest compact regions in the final stages of collapse towards the formation of new stars.

Prof Richard Davis of the University of Manchester's Jodrell Bank Centre for Astrophysics says "the real power of Planck is the combination of the High and Low Frequency Instruments which allow us, for the first time, to disentangle the three foregrounds. This is of interest in its own right but also enables us to see the Cosmic Microwave Background far more clearly".

Once formed, the new stars disperse the surrounding gas and dust, changing their own environment. A delicate balance between star formation and the dispersion of gas and dust regulates the number of stars that any given galaxy makes. Many physical processes influence this balance, including gravity, the heating and cooling of gas and dust, magnetic fields and more. As a result of this interplay, the material rearranges itself into 'phases' which coexist side-by-side. Some regions, known as 'molecular clouds', contain dense gas and dust, while others, referred to as 'cirrus', contain more diffuse material. Investigating the properties of the different components in the interstellar medium requires data at a range of frequencies. Planck will advance this effort hugely because it provides, for the first time, data on all the main emission mechanisms in one go. Planck’s wide wavelength coverage, which is required to study the Cosmic Microwave Background, proves also to be crucial for the study of the interstellar medium.

Mapping the Sites of Star Formation

The power of multi-frequency observations to discern the processes at play during star formation is beautifully demonstrated by these new Planck images. This first sequence, shown on the left below, shows the interstellar medium in a region of the Orion Nebula where stars are actively forming in large numbers. The comparable sequence of images on the right, showing a region where fewer stars are forming near the constellation of Perseus, illustrates how the structure and distribution of the interstellar medium can be distilled from the images obtained with Planck.

Orion as seen in three of Planck's wavelength channels (left panels), and as a three-colour image. Click here, or on the image, for a high resolution version. Also available are the individual channels: 1cm (30 GHz, red), 1mm (353 GHzgreen) and 0.3mm (blue). Image credit: ESA / HFI and LFI Consortia.

Orion as seen in three of Planck's wavelength channels (left panels), and as a three-colour image. Click here, or on the image, for a high resolution version. Also available are the individual channels: 1cm (30 GHz, red), 1mm (353 GHz, green) and 0.3mm (857 GHz, blue). Image credit: ESA / HFI and LFI Consortia.

Professor Peter Ade of Cardiff University, and co-Investigator on Planck’s High Frequency Instrument, said “the power of Planck’s very wide wavelength coverage is immediately apparent in these images. The red loop seen in the Orion image is Barnard’s Loop, and the fact that it is visible at longer wavelengths tells us that it is emitted by hot electrons, and not by interstellar dust. The ability to separate the different emission mechanisms is key for Planck’s primary mission”.

A lucrative by-product for a ground-breaking mission

Precise measurements of the Cosmic Microwave Background are crucial to cosmology, and will improve our understanding of how our Universe formed and evolved. Planck will measure the Cosmic Microwave Background at the highest-sensitivity (a few parts per million), and resolution (5 arcminutes) over the whole sky. Doing so requires the removal of the 'foreground' emission arising from the Milky Way. The information gleaned during this process is providing, as a by-product, a unique view of the processes that led to the formation of the stars in the galaxies that populate our Universe. Since Planck will observe the Cosmic Microwave Background over the whole sky, it can also map large regions of the Galaxy. The Orion image is 13 degrees across (26 times the width of the full moon as seen from Earth), while the Perseus image is 30 degrees across (60 times the width of the full moon). The image on the left shows the Planck images of the Orion region (on the left) and the Perseus region (on the right) overlaid on a picture of the sky at visible and infrared wavelengths.

Dr Clive Dickinson, also of the University of Manchester, commented that "the Planck maps are really fantastic to look at. These are exciting times."

The images below show the same regions compared with their appearance in visible light.

Part of the constellation of Orion as seen in visible light (top), and by Planck (bottom). The image shows the three stars in the hunter's belt (left of centre) and his sword (below centre). Barnard's loop is seen clearly in both images. The Orion Nebula is much more extensive when seen by Planck, and fills much of the image. Click on the images for higher resolution versions. Image credit: STSci/DSS (top), ESA / HFI and LFI Consortia (bottom).

A region of the sky centred on the constellation of Perseus as seen in visible light (top), and by Planck (bottom). The image shows the California Nebula (centre) and the Severn Sisters, of Pleiades (bottom left). While this regions is relatively empty in visible light, it is laced through with cold dust when seen by Planck. Click on the images for higher resolution versions. Image credit: STSci/DSS (top), ESA / HFI and LFI Consortia (bottom).

Michael Eugene Adams has created animations of the Planck images, which can be viewed on YouTube. Below is the animation of the Orion image, and the equivalent for the Perseus region can be viewed here.